There are many uses for induction motors where it is required or desirable to variably control the motor's speed in response to a sensed condition that is related to the particular task being performed by the motor. A.C. induction motors have long been favored for use in industrial applications, due to their simplicity, cost-effectiveness and high reliability. The A.C. induction motor is basically a fixed speed motor. However, since its speed of operation depends upon the frequency of its energizing power, the induction motor can be made to operatively function as a variable speed motor by varying the frequency of its energizing source.
A number of various techniques for providing a variable frequency drive to an A.C. induction motor are known. Solid state technology has provided a number of such devices, commonly referred to as solid state adjustable frequency inverters. An example of such a solid state inverter, sold under the Ampli-Cycle.RTM. tradename and mark and owned by the common assignee of this invention, is disclosed in U.S. Pat. No. 3,748,556, issued on July 24, 1973, to Gillett. Such solid state adjustable frequency inverters generally provide output pulses of voltage and current which are electronically formed into simulated sine waves, which are used to directly energize the A.C. induction motor.
Variable speed induction motor networks have been used in a number of industrial applications, in both open and closed-loop systems, wherein the speed of the motor is varied in response to a sensed condition. In an open-loop system, the sensed condition is merely monitored, and perhaps recorded, and the motor speed is manually adjusted in response to the monitored signal. In a closed-loop system, a control network is interposed between the sensor and the motor, for directly controlling the motor speed responsive to the sensed condition.
Use of the variable speed induction motor has been found to be particularly advantageous in driving pumping apparatus for use in industrial pumping applications, wherein it is desirable to control or to regulate the speed of pumping in response to variable sensed conditions, either from the source of liquid being pumped or from the reservoir into which the liquid is being pumped. Typically, in such applications, the time constants involved (i.e. the length of time required for the pumping apparatus to effect the desired change in the controlled environment, following a sensed undesired condition) are relatively short. Accordingly, for example, in the pumping of sewage, extreme control accuracy over the speed of the induction motor is not required. Oftentimes in such applications, a simple open-loop system will suffice, wherein the operator of the system can manually adjust the frequency of the induction power source to effect the desired change in pumping speed.
However, control of pumping speed, and thus the accuracy of controlling the input power frequency for an induction motor, is extremely important when the induction motor is used to drive a submersible pump for pumping oil from deep oil wells. In such applications, the induction motor itself is located within the casing near the bottom of the well (oftentimes as much as a mile below the ground surface), and is itself submersed within the oil medium being pumped from the casing.
The "artificial lift" or "pumping" of oil from an oil well is employed in all phases of "secondary" and "tertiary" recovery applications, and is also sometimes used in the "primary" recovery of oil from the well. A number of different methods, besides usage of the induction motor operated pump method, have been used for such "artificial lift" oil pumping applications. Due in large part to its mechanical simplicity and reliability, the well-known "rod pumping" technique has been the most common method employed for artificially pumping oil from an oil well.
The rod pump basically comprises a mechanical pump located near the bottom of the well casing at the level of the oil reservoir, which is activated by a sucker rod that extends from the pump, through the casing, and all the way up to the well head at the ground surface. The top of the sucker rod is secured to a "walking beam" (rocker arm) that moves up and down through the force of a crank and counterweight powered by a prime mover. As the sucker rod moves up and down, the mechanical pump is activated to lift oil from the reservoir and up to the surface. The most critical element of the rod pump is the sucker rod, which mechanically extends thousands of feet in length, and tends to operate like a flexible spring. Accordingly, significant stresses can develop in the rod, causing metal fatigue and breakage thereof, leading to substantial down time for repairs. Further, due to the mechanical linkage requirements of the rod pump, it can be used only in wells having straight casings (i.e. wherein the oil is accessible only through a straight hole).
Other artificial lift techniques for removing oil from a well include the "gas lift" and "hydraulic pumping" techniques. The gas lift technique is useful primarily in association with wells having a readily available supply of natural gas. This technique basically consists of forcing gas into the well casing, and providing a series of gas lift valves along the tubing in a manner such that the gas forces the oil into the tubing and to the well head. This method is practical for use only with a naturally available source of gas. The hydraulic pumping technique employs a pump located at the reservoir level which is driven by hydraulic means, rather than by a mechanical motor. Typically the hydraulic drive is provided by crude oil which has already been removed from the well, which is forced down the tubing, under pressure, to activate the hydraulic motor. This technique makes testing of the well difficult, since both power supplying oil and the new oil to be pumped are present in the well stream.
Use of the induction motor pumping technique is rapidly gaining acceptance and is among the fastest growing artificial lift methods used in oil production today. Because the induction motor pump receives its power through flexible cable, rather than through a sucker rod, this pump is appropriate for installation in deviated wells, or other wells not accessible through a straight hole. Further, the induction motor pump is capable of producing very high volumes of oil from the well.
Prior art techniques for recovering oil by induction motor pumping techniques have suffered from serious problems, due in large measure to the inability of prior art induction motor pumping systems to accurately control and to optimize the pumping speed in response to the changing pressure conditions within the oil well reservoir.
Due to the large distance required to lift the oil from the reservoir to the top of the well, significant back-pressure due to the weight of the column of oil being lifted is applied to the submersed pump. Accordingly, the induction motor operating the pump must be powerful enough to perform the heavy lifting operation. The ability of the pump to perform the required pumping operation, and therefore also the power requirements for energizing the induction motor of the pump, will depend in large measure upon the pressure exerted by the oil reservoir within the casing upon the pump. It is highly desirable to maintain the oil reservoir pressure within the casing as constant as possible, to optimize the efficiency of pumping from the well. For example, if a decrease in pressure within the oil reservoir occurred, and was not responded to soon enough, the speed of the pump may be too fast, causing the level of oil within the reservoir to drop below the level of the pump, causing gasification of the oil being pumped, and possible damage to the pump by overheating due to lack of cooling from the normally surrounding oil reservoir. Such a condition can be extremely costly, should the pump be required to be removed for repair. On the other hand, should an increase in oil reservoir pressure, due to changing oil field conditions, go unnoticed, the pump would not be operating at its maximum pumping efficiency, since with the increased reservoir pressure, the pumping speed could be increased.
Control of the motor speed, however, in such oil recovery applications, has heretofore not been performed with a closed-loop system, due in large measure to the long time constant associated with such pumping techniques. Such long time constants, also make control of such an induction motor pumping system by analog techniques impractical. Accordingly, digital pressure and temperature sensing methods for sensing the temperature and pressure of the oil reservoir within the casing have been used. An example of such a digital pressure and temperature sensor which has been successfully used with such oil well pumping applications is described in U.S. Pat. Nos. 3,968,691 issued on July 13, 1976, and 4,078,232 issued on Mar. 7, 1978, both to Balkanli.
While the digital pressure and temperature sensing techniques described by the Balkanli patents offer a significant improvement in the art of measuring temperature and pressure within an oil well reservoir, the accuracy of control of the submersed induction motor pump has heretofore been limited by the digital accuracy of the sensing device. For example, if the lowest increment of measurement recordable by the digital sensor were 10 p.s.i., control of the induction motor speed so as to maintain the oil reservoir pressure at a desired pressure level, was virtually impossible. In the prior art open-loop system of controlling the induction motor speed in response to a sensed reading from the digital sensor, an operator would have to guess at the appropriate change in speed level required in the system. After effecting the change in motor speed, due to the long time constant involved before recognition of the effect of the changed speed could be realized, the operator would have no way of determining whether his change in speed guess had been correct until the system had had an adequate time in which to stabilize to the changed conditions. Such techniques involved significant operator attention, and resulted in considerable oscillation of pressures within the oil well reservoir, due to "overshooting" and "undershooting" of the optimum pressure level, due to the guessing technique employed at controlling the motor speed.
The present invention solves the hit-and-miss guessing problems of the prior art techniques associated with control of induction motor pump speeds, by providing a control apparatus and method for regulating the speed of an induction motor pump so as to maintain the oil reservoir pressure at a level of accuracy "greater" than the accuracy of pressure measurement provided by the sensor itself. The control apparatus and method of this invention accurately predict, based on past operative conditions of the system over a period of time, the proper changes in motor speed currently required to effect the desired operational condition being sensed. Applicability of the present invention for use in pumping operations for oil and other mediums, and generally, to the closed-loop control of an induction motor speeds in response to a sensed variable parameter, will be readily apparent upon a more detailed description of the present invention.